# Django 2.0.dev20170216164632 documentation

# Source code for django.db.models.indexes

    import hashlib

    from django.utils.encoding import force_bytes

    __all__ = ['Index']

    # The max length of the names of the indexes (restricted to 30 due to Oracle)
    MAX_NAME_LENGTH = 30

    class Index:
        suffix = 'idx'

        def __init__(self, fields=[], name=None):
            if not isinstance(fields, list):
                raise ValueError('Index.fields must be a list.')
            if not fields:
                raise ValueError('At least one field is required to define an index.')
            self.fields = fields
            # A list of 2-tuple with the field name and ordering ('' or 'DESC').
            self.fields_orders = [
                (field_name[1:], 'DESC') if field_name.startswith('-') else (field_name, '')
                for field_name in self.fields
            ]
            self.name = name or ''
            if self.name:
                errors = self.check_name()
                if len(self.name) > MAX_NAME_LENGTH:
                    errors.append('Index names cannot be longer than %s characters.' % MAX_NAME_LENGTH)
                if errors:
                    raise ValueError(errors)

        def check_name(self):
            errors = []
            # Name can't start with an underscore on Oracle; prepend D if needed.
            if self.name[0] == '_':
                errors.append('Index names cannot start with an underscore (_).')
                self.name = 'D%s' % self.name[1:]
            # Name can't start with a number on Oracle; prepend D if needed.
            elif self.name[0].isdigit():
                errors.append('Index names cannot start with a number (0-9).')
                self.name = 'D%s' % self.name[1:]
            return errors

        def get_sql_create_template_values(self, model, schema_editor, using):
            fields = [model._meta.get_field(field_name) for field_name, order in self.fields_orders]
            tablespace_sql = schema_editor._get_index_tablespace_sql(model, fields)
            quote_name = schema_editor.quote_name
            columns = [
                ('%s %s' % (quote_name(field.column), order)).strip()
                for field, (field_name, order) in zip(fields, self.fields_orders)
            ]
            return {
                'table': quote_name(model._meta.db_table),
                'name': quote_name(self.name),
                'columns': ', '.join(columns),
                'using': using,
                'extra': tablespace_sql,
            }

        def create_sql(self, model, schema_editor, using=''):
            sql_create_index = schema_editor.sql_create_index
            sql_parameters = self.get_sql_create_template_values(model, schema_editor, using)
            return sql_create_index % sql_parameters

        def remove_sql(self, model, schema_editor):
            quote_name = schema_editor.quote_name
            return schema_editor.sql_delete_index % {
                'table': quote_name(model._meta.db_table),
                'name': quote_name(self.name),
            }

        def deconstruct(self):
            path = '%s.%s' % (self.__class__.__module__, self.__class__.__name__)
            path = path.replace('django.db.models.indexes', 'django.db.models')
            return (path, (), {'fields': self.fields, 'name': self.name})

        @staticmethod
        def _hash_generator(*args):
            """
            Generate a 32-bit digest of a set of arguments that can be used to
            shorten identifying names.
            """
            h = hashlib.md5()
            for arg in args:
                h.update(force_bytes(arg))
            return h.hexdigest()[:6]

        def set_name_with_model(self, model):
            """
            Generate a unique name for the index.

            The name is divided into 3 parts - table name (12 chars), field name
            (8 chars) and unique hash + suffix (10 chars). Each part is made to
            fit its size by truncating the excess length.
            """
            table_name = model._meta.db_table
            column_names = [model._meta.get_field(field_name).column for field_name, order in self.fields_orders]
            column_names_with_order = [
                (('-%s' if order else '%s') % column_name)
                for column_name, (field_name, order) in zip(column_names, self.fields_orders)
            ]
            hash_data = [table_name] + column_names_with_order + [self.suffix]
            self.name = '%s_%s_%s' % (
                table_name[:11],
                column_names[0][:7],
                '%s_%s' % (self._hash_generator(*hash_data), self.suffix),
            )
            assert len(self.name) <= 30, (
                'Index too long for multiple database support. Is self.suffix '
                'longer than 3 characters?'
            )
            self.check_name()

        def __repr__(self):
            return "<%s: fields='%s'>" % (self.__class__.__name__, ', '.join(self.fields))

        def __eq__(self, other):
            return (self.__class__ == other.__class__) and (self.deconstruct() == other.deconstruct())